The present invention relates to a fuel cell separator having high strength and low modulus and being superior in flexibility. The present invention relates also to a fuel cell of solid polymer type having good vibration resistance and shock resistance. Said fuel cell separator is suitable for fuel cells as the mobile power source for automobiles, hybrid cars, and small ships. Said fuel cell of solid polymer type employs said fuel cell separator for all or part of its fuel cell separators.
A fuel cell generates electricity directly from fuel (such as hydrogen) and oxygen (in the atmosphere) supplied to it through electrochemical reactions to form water. It is capable of efficient energy conversion and free from environmental pollution. Thus it is finding new uses in various applications as small-scale local power source, domestic power source, simple power source at camp sites, mobile power source (for automobiles, hybrid cars, and small ships), and special power source (for artificial satellites and space development).
A fuel cell system, particularly that of solid polymer type, consists of tens or hundreds of unit cells which are stacked to form the battery module. As shown in FIG. 1, each unit cell is made up of one electrolytic membrane of solid polymer 2, two gas diffusion electrodes of carbon paper 3, 3, and two flat separators 1, 1, each having, on both sides thereof, ribs 1a which form channels 4 for gas (such as hydrogen and oxygen) to be supplied and discharged.
The fuel cell separator mentioned above is required to have high electrical conductivity, low gas permeability, and electrochemical stability so that it imparts electrical conductivity to individual unit cells and functions as channel for fuel and oxygen (air) and also as a separating membrane. Moreover, each unit cell is low in output voltage, which makes it necessary to stack tens or hundreds of unit cells in order to construct a fuel cell system with a practical capacity of the order of 100 kW. This has caused a demand for fuel cell separators having good thickness and surface accuracy and dimensional stability. This accuracy is necessary for good communication between the separator and the gas diffusion electrode.
The fuel cell separator has been made of metallic material (such as stainless steel and titanium) or carbonaceous material (such as vitreous carbon). However, none of them are satisfactory in performance and price.
There has recently been proposed a fuel cell separator formed from a carbonaceous material composed mainly of thermosetting resin and graphite particles. This fuel cell separator needs a large amount of graphite particles to impart an adequate level of conductivity. Graphite particles contribute to conductivity and strength but make the fuel cell separator less flexible. Fuel cell separators with low flexibility are liable to cracking when they are tightened during fuel cell assembling or when fuel cells receive shocks or vibration during their use as mobile power sources for automobiles, hybrid cars, or small ships.
The present invention was completed in view of the foregoing. Accordingly, it is an object of the present invention to provide a high-performance fuel cell separator and a fuel cell of solid polymer type. Said fuel cell separator is characterized by low modulus, high strength (which prevents damage during assembling), high electrical conductivity, high gas impermeability, high electrochemical stability, and high dimensional stability, which are required of fuel cell separators. Said fuel cell is partly or entirely provided with said fuel cell separator, and hence it is characterized by good vibration and shock resistance.
In order to achieve the above-mentioned object, the present inventors carried out extensive studies on relation between flexural strength and distortion in the fuel cell separator formed from a compound composed mainly of thermosetting resin and graphite particles. As the result, it was found that the fuel cell has good flexibility when it has high strength as well as low flexural modulus (which permits a large amount of bending).
FIG. 3 shows the relation between flexural strength and bending. Comparative Example 1 has a high flexural strength and a low bending, and Comparative Example 2 has a low flexural strength and a low bending. They have a considerably high flexural modulus which is a ratio of flexural stress to strain within the limit of elasticity of the material, which is shown as the slope of the graph in FIG. 3. Therefore, they are less flexible than Examples 1 and 2. This result suggests that the fuel cell separator should have a high flexural strength and a low flexural modulus (leading to a large bending) if it is to have good flexibility, high thickness accuracy, surface accuracy, and good vibration and shock resistance.
A fuel cell separator of conventional type is formed from a compound composed mainly of thermosetting resin and graphite particles. It decreases in conductivity and increases in strength as graphite decreases in average particle diameter, whereas it increases in conductivity and decreases in strength as graphite increases in average particle diameter. Both high conductivity and high strength are attained by incorporation with an adequate amount of graphite having a proper average particle diameter. The present inventors found that this conventional practice is not satisfactory for the fuel cell separator to have high flexural strength and low flexural modulus (that permits more bending). It was found that the object is effectively achieved not only by adjusting the amount of graphite and the average particle diameter of graphite but also by using graphite with a high ratio of coarse particles and a broad particle size distribution (difference between the maximum particle diameter and the minimum particle diameter).
In FIG. 4, Comparative Examples 1 to 3 show results in the case of graphite having a narrow particle size distribution (difference between the maximum particle diameter and the minimum particle diameter), particularly Comparative Examples 1 and 2 show results in the case of graphite of flaky particles. By contrast, Examples 1 and 2 show results in the case of graphite of needlelike particles having a broad particle size distribution (with a large maximum particle size and a high ratio of large particles). It is apparent from FIG. 3 that products in Comparative Examples 1 to 3 are inferior in flexural strength, flexural modulus, and distortion to those in Examples 1 and 2. This suggests that high flexural strength and low flexural modulus (increased amount of distortion) are effectively achieved if graphite has a large maximum particle diameter and a broad particle size distribution.
After their continued studies based on the above-mentioned finding, the present inventors found that it is possible to produce a satisfactory fuel cell separator (having on one side or both sides thereof grooves for gas supply and discharge) from a compound composed mainly of thermosetting resin and graphite particles only when the graphite particles (preferably needlelike graphite particles) have an average particle diameter of 20-100 xcexcm and a maximum particle diameter of 240 to 550 xcexcm and hence has a broad particle size distribution (difference between the maximum particle diameter and the minimum particle diameter). The resulting fuel cell separator has a flexural modulus of M1 GPa and a flexural strength of M2 MPa (both measured according to JIS K6911) which satisfy the following relations
xe2x80x83900xe2x89xa6M1xc3x97M2xe2x89xa62000
2xe2x89xa6M2/M1xe2x89xa610
and also has a flexural modulus equal to or equal to or lower than 20 GPa and a flexural strength equal to or higher than 50 MPa. The fuel cell separator superior in flexibility does not crack due to distortion at the time of assembling into a fuel cell, nor does it break when it receives strong vibrations and shocks because it absorbs them. If this fuel cell separator is used partly or entirely as the fuel cell separator of fuel cell of solid polymer type, the resulting fuel cell has good gas sealability and good shock and vibration resistance. Hence it is suitable for use as mobile electric sources for automobiles, hybrid cars, and small ships.
The present invention provides a fuel cell separator and a fuel cell of solid polymer type which are defined in the following.
(1) A fuel cell separator having on one side or both sides thereof channels for gas supply and discharge formed from a compound composed mainly of thermosetting resin and graphite particles, characterized in having a flexural modulus of M1 GPa and a flexural strength of M2 MPa (both measured according to JIS K6911) which satisfy the following relations.
900xe2x89xa6M1xc3x97M2xe2x89xa62000
2xe2x89xa6M2/M1xe2x89xa610
(2) A fuel cell separator having on one side or both sides thereof channels for gas supply and discharge formed from a compound composed mainly of thermosetting resin and graphite particles, characterized in having a flexural modulus equal to or lower than 20 GPa and a flexural strength equal to or higher than 50 MPa (both measured according to JIS K6911).
(3) A fuel cell separator having on one side or both sides thereof channels for gas supply and discharge formed from a compound composed mainly of thermosetting resin and graphite particles, characterized in that above-mentioned graphite particles have an average particle diameter of 20 to 100 xcexcm and a maximum particle diameter of 240 to 500 xcexcm.
(4) A fuel cell separator as defined in (3) above, wherein above-mentioned graphite particles have the following particle size distribution.
(5) A fuel cell separator as defined in (3) or (4) above, wherein the graphite particles are needle-like graphite particles.
(6) A fuel cell separator as defined in any of (3) to (5) above, wherein the graphite particles are added in an amount of 200 to 900 pbw for 100 pbw of the thermosetting resin.
(7) A fuel cell separator, as defined in any of (3) to (6) above, having on one side or both sides thereof channels for gas supply and discharge formed from a compound composed mainly of thermosetting resin and graphite particles, characterized in having a flexural modulus of M1 GPa and a flexural strength of M2 MPa (both measured according to JIS K6911) which satisfy the following relations.
900xe2x89xa6M1xc3x97M2xe2x89xa62000
2xe2x89xa6M2/M1xe2x89xa610
(8) A fuel cell separator, as defined in any of (3) to (6) above, having on one side or both sides thereof channels for gas supply and discharge formed from a compound composed mainly of thermosetting resin and graphite particles, characterized in having a flexural modulus equal to or lower than 20 GPa and a flexural strength equal to or higher than 50 MPa (both measured according to JIS K6911).
(9) A fuel cell system of solid polymer type constructed of a number of unit cells, each consisting of an electrolytic membrane of solid polymer, a pair of electrodes holding above-mentioned membrane between them, and a pair of separators holding above-mentioned electrodes between them in such a way as to form channels for gas supply and discharge, characterized in that the fuel cell separator defined in any of (1) to (8) above is used for part or all of the separators in above-mentioned fuel cell system.